Lambda trigger on stream events in DynamoDB - Step-by-Step Execution

Concept Flow - Lambda trigger on stream events

DynamoDB Table Update

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Stream Records Created

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Lambda Triggered

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Lambda Processes Records

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Lambda Executes Business Logic

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Optional: Write Back to DB or Other Service

When a DynamoDB table changes, a stream record is created. This triggers a Lambda function that processes the record and runs your code.

Execution Sample

DynamoDB

1. Update DynamoDB item
2. Stream captures change
3. Lambda triggered with event
4. Lambda reads event records
5. Lambda processes each record

Shows how a DynamoDB update triggers a Lambda function via stream events.

Execution Table

Step	Event Source	Action	Lambda Input	Lambda Output
1	DynamoDB Table	Item updated	N/A	N/A
2	DynamoDB Stream	Stream record created	Record with change info	N/A
3	Lambda Service	Lambda triggered	Event with stream records	N/A
4	Lambda Function	Reads event records	Stream records array	Parsed records
5	Lambda Function	Processes each record	Single record	Business logic result
6	Lambda Function	Completes execution	N/A	Success or error logged
7	End	No more records	N/A	Lambda idle until next event

💡 No more stream records to process, Lambda waits for next trigger

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 5	Final
streamRecords	empty	contains 1 record	parsed 1 record	processed 1 record	empty after processing
lambdaEvent	none	none	event with records	event processed	none

Key Moments - 2 Insights

Why does Lambda receive an event with multiple records sometimes?

Because DynamoDB streams batch multiple changes before triggering Lambda, as seen in execution_table step 3 and 4 where Lambda input is an event with an array of records.

What happens if Lambda fails to process a record?

Lambda logs an error and may retry depending on configuration. This is implied in execution_table step 6 where Lambda completes execution with success or error logged.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does Lambda first receive the stream event?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Check the 'Lambda Input' column to see when Lambda is triggered with the event

According to variable_tracker, what is the state of 'streamRecords' after Lambda processes each record?

A parsed 1 record

B contains 1 record

C empty after processing

D none

💡 Hint

Look at the 'After Step 5' and 'Final' columns for 'streamRecords'

If multiple items update quickly, how does Lambda receive the events?

A Batch of records in one event

B No events triggered

C One event per item update

D Only last update triggers event

💡 Hint

Refer to key_moments explanation about batching and execution_table steps 3 and 4

Concept Snapshot

Lambda triggers on DynamoDB stream events when table data changes.
Stream batches changes and sends records to Lambda.
Lambda receives event with array of records.
Lambda processes each record in order.
Errors can cause retries.
Useful for real-time processing of DB changes.

Full Transcript

When you update a DynamoDB table, the change creates a stream record. This stream record triggers a Lambda function. The Lambda function receives an event containing one or more stream records. It reads and processes each record, running your business logic. After processing, Lambda finishes and waits for the next trigger. Sometimes multiple changes batch into one event, so Lambda handles multiple records at once. If Lambda fails, it logs errors and may retry. This setup lets you react instantly to database changes with your code.